Many hours are spent in heated debate on this topic, but the truth is that the number of reps per set and number of sets per workout are secondary measures. A close review of the scientific research shows that the weight of evidence does NOT support the idea that different numbers of repetitions have differential effects on muscular strength and endurance. Furthermore, a review on this subject concluded that “all these studies strongly suggest that within a reasonable range of repetitions, approximately 3 to 20, there does not appear to be a specific number of repetitions that will elicit more favorable gains in muscular strength, power or hypertrophy.”
What does matter then? It is ALL about reaching momentary muscular failure (meaning that you cannot complete another repetition of an exercise in good form. If you do not reach muscular failure or come very close the reps and sets will not produce results. Perfect form is part of this equation because perfect form means keeping tension in the targeted muscle/s throughout the entire repetition and not using momentum. For example, allowing the weights being lifted to settle on the top of the weight stack means you are unloading the muscle – NOT what we want to do! Throwing weight using momentum is another way you can deload the muscle. The key is performing smooth, steady repetitions throughout the range of motion for an exercise with no deloading until you reach momentary muscular failure.
For beginners one set to failure between 8 – 15 repetitions is plenty for each muscle group. Over the longer haul repetitive sets because necessary to make progress – but never sacrifice reaching muscular failure and perfect form to do extra sets!